The invention relates to steel sheets provided with a coating comprising a main layer of zinc-chromium alloy, the predominant phase of which is xcex4 or xcex6.
The invention also relates to the plant and the process for obtaining steel sheets coated with a coating of this type of alloy.
Document EP 0 607 452 (KAWASAKI) describes the crystalline phases that can be obtained by depositing monolayer coatings of Znxe2x80x94Cr alloys by electrodeposition and the main properties associated with these phases; depending on the respective proportions of zinc and chromium and depending on the electrodeposition conditions, the following main alloy phases may therefore be distinguished:
xcex7, having a hexagonal structure identical to that of pure zinc, in which the chromium is in solid solution in small proportions;
xcex4, also having a hexagonal structure, the cell parameter a of which is greater than that of the xcex7 structure, the cell parameter c of which is less than that of the xcex7 structure;
xcex93, having a body-centred cubic structure, the cell parameter of which is greater than that of pure chromium.
Document JP 08-013192 A (KAWASAKI) teaches that annealing a layer of zinc-chromium alloy, the phase of which is xcex4, transforms the structure of this layer; under treatment conditions which vary depending on the chromium content, the annealing time and the annealing temperature (130xc2x0 C. to 200xc2x0 C.), what is obtained is:
a xcex6 phase, having a monoclinic structure.
FIG. 5 shows the diagram, established by the Applicant, of the amalgamation of the trials for the identification and thermostability of the phases of the Znxe2x80x94Cr alloys as a function of the chromium content (%) in the particular case of coatings deposited under vacuum by PVD (Physical Vapour Deposition).
On page 9, lines 57-58 of document EP 0 607 452, it is mentioned that, below a 5% chromium weight content in the alloy, the xcex93 phase will not form in the coating and that, above 30% chromium, the coating obtained does not exhibit good adhesion to the steel.
This information would therefore dissuade someone from using high chromium contents, greater than 30%, especially at the interface between the steel and the coating, in order to avoid impairing the adhesion.
According to that disclosure, in which a layer of zinc-chromium alloy adheres poorly to the steel if the interface between this layer and the substrate is too rich in chromium, it has actually been found that the layers of zinc-chromium graded alloy, produced under vacuum, having a chromium concentration of approximately 15% at the interface and of approximately 5% at the surface, posed problems of adhesion to the steel substrate; by bending a steel sheet coated with such a layer through 180xc2x0, a partial debonding of the alloy layer is in fact observed.
The object of the invention is to remedy this drawback.
For this purpose, the subject of the invention is a steel sheet provided with a coating comprising a main layer of zinc-chromium alloy, the predominant phase of which has a xcex4 and/or xcex6 structure, characterized in that the said coating also includes a subjacent adhesion layer made of zinc-chromium alloy, sandwiched between the steel of the sheet and the said main layer, which has:
a xcex93-type body-centred cubic crystal structure;
a chromium weight content high enough to obtain the said xcex93 structure;
an at least partial epitaxial junction with the said steel, manifested by the presence of incomplete rings in an electron diffraction pattern of the said sublayer, produced on sections made near the interface with the steel and parallel to this interface.
The invention may also have one or more of the following features:
the chromium weight content in the said subjacent layer is between 30 and 70%;
the thickness of the said subjacent layer is between 0.01 xcexcm and 1 xcexcm;
the thickness of the said main layer is greater than 1 xcexcm; this is because the roughness of the substrate is often of the order of 1 xcexcm and sometimes greater; in order for the coating to provide effective protection against corrosion and good resistance to red rust, it is important that the coating be sufficiently covering and that its thickness be greater than the roughness of the substrate;
the said main layer is a layer with a chromium concentration gradient; preferably, the variation in chromium concentration through the thickness of the said main layer is greater than or equal to 10% by weight;
the chromium concentration in the said main layer may be higher near the surface than near the steel;
preferably, in order to obtain both effective protection against corrosion and good phosphatizability, the chromium concentration in the said main layer is higher near the steel than near the surface;
the predominant phase of the main layer has xcex6 structure;
the main layer of the coating may only partially have a xcex6 structure;
in particular, if the coating is exposed to X-radiation at a grazing angle of incidence of about 3xc2x0, the said radiation emanating from an X-ray tube with a cobalt anticathode supplied with 30 kV and outputting 30 mA, emitting a line of wavelength Ka=0.179026 nm, and if the diffraction of this radiation is analysed using a rear monochromator and a scintillation detector, the height of the (131) diffraction line of the xcex6-ZnCr phase is between approximately 10% and 100% of the height of the (0002) line of the xcex4-ZnCr phase.
The subject of the invention is also a process for manufacturing a sheet according to the invention from a sheet to be coated, characterized in that it comprises a step in which the said coating is applied by vacuum deposition to the surface of the said sheet to be coated.
The invention may also have one or more of the following features:
the said coating is applied by vacuum evaporation and/or sublimation of zinc and chromium;
the said sheet to be coated is heated during the deposition to a temperature of between approximately 170xc2x0 C. and 230xc2x0 C.;
immediately before the deposition step, the said surface is cleaned and/or brightened suitably for obtaining the said at least partial epitaxial junction, preferably by inert-gas ion bombardment.
The invention also relates to the plants for obtaining, continuously, coatings of alloys, essentially comprising two metal elements, on a sheet, and more particularly coatings of Znxe2x80x94Cr alloy, by a process for the vacuum deposition of these elements, in which the said sheet is made to run continuously, in succession, past a source of the first alloy metal element and then past a source of the second metal alloy element.
The word xe2x80x9csourcexe2x80x9d in this text may denote an evaporation source or a sublimation source which may be heated by electron bombardment, by conduction (resistance or induction heating), by radiation or by plasma.
In such a process, when the flow rate of vapour emitted by the metal source is low ( less than 1 g/min.cm2), the metal vapour pressure gradient above the source is low and the evaporated or sublimed metal atoms consequently undergo very few collisions. It may therefore be considered that the great majority of atoms propagate between the source and the substrate in a straight line.
On the other hand, if the flow rate of vapour emitted by the metal source is high ( greater than 1 g/min.cm2), the metal vapour pressure gradient above the source is high and a great majority of the metal atoms which leave the source will undergo several collisions before reaching the substrate. The dispersion of the metal element over the substrate will accordingly be greater the higher the vapour flow rate.
It is known that, by placing a screen between the source and the substrate seen from this source at a given solid angle, some of the vapour emitted in this solid angle is stopped by this screen and no longer condenses on the substrate. If the vapour flow rate is low and if the residual pressure in the deposition chamber is low, between 10xe2x88x924 and 10xe2x88x921 Pa, it is practically all of the vapour which will be stopped.
When a strip to be coated runs in succession past an evaporation or sublimation source of a first alloy element A and then past an evaporation or sublimation source of a second alloy element B, as described for example in document JP 06 212410 A (ISHIKAWAJIMA HARIMA HEAVY IND Ltd), the alloy layer obtained is not homogeneous in terms of composition through the depth: richer in element A closer to the substrate than at the surface, richer in element B at the surface than close to the substrate; the alloy layer obtained is termed a xe2x80x9cgradedxe2x80x9d layer.
Document JP 03-191053 (KOBE STEEL) discloses a graded Znxe2x80x94Cr coating on a steel sheet, in which the chromium concentration at the interface with the substrate is greater than 10% while it is less than 5% at the external surface; such a coating provides both significant corrosion resistance, because of the high chromium concentration in the depth, and good phosphatizability, because of the low chromium concentration at the surface.
To obtain such a graded Znxe2x80x94Cr coating using the aforementioned process, i.e. richer in chromium on the substrate side than at the surface, it is necessary to make the strip to be coated run firstly past the chromium source and then past the zinc source; in general, to obtain a graded coating of alloy AB, richer in element A on the substrate side than at the surface, it is necessary to make the substrate run firstly past the source A and then past the source B.
The vacuum deposition plants comprise:
means for running the sheet or strip of sheet;
an evaporation and/or sublimation chamber open to an evaporation and/or sublimation window emerging in a region of the path along which the sheet runs;
at least one source of elements to be deposited, placed in this chamber so as to emit through the window.
The useful solid angle of emission from a source is in general bounded by this window; it encompasses the possible trajectories of elements liable to condense on the substrate.
In the deposition plants comprising a source of element A and a source of element B for producing a coating comprising a layer of alloy AB, the A and B sources are placed in the same chamber so as to emit through the same window; if the A and B sources are placed at the same distance from this window, the useful angle of emission from each element is identical.
In the case of plants suitable for producing graded layers in which the strip to be coated runs in succession firstly past an A source and then past a B source which emit through the same window, document JP 06 212410 A, already mentioned, describes the use of screens, xe2x80x9cmasksxe2x80x9d or xe2x80x9cstopsxe2x80x9d, in order to obtain multilayer coatings comprising a graded layer of alloy AB and a layer of pure element A and/or a layer of pure element B.
Processes for obtaining such multilayer coatings using these plants will now be described with reference to FIGS. 1 to 3: the arrow x indicates the running direction of the substrate to be coated.
The window through which the A source and the B source emit have two edges; these edges are positionally adjustable (i.e. vertically and/or laterally) and thus form stops; the edge which is located on the A source side is called the xe2x80x9cA entry edgexe2x80x9d and the edge which is located on the B source side is called the xe2x80x9cB exit edgexe2x80x9d.
Referring to FIG. 2, if the A entry edge is placed at a height hx from the path along which the strip runs (and not near it), upon entering the deposition zone axe2x80x2axe2x80x3, the element B will be mostly deposited, and a xe2x80x94B(+A) xe2x80x94AB-type bilayer coating will be obtained, in which the B(+A) sublayer will have an A content which will depend mainly on the flow rate of vapour from the source and on the residual pressure in the deposition chamber and in which the AB layer is richer in element A on the substrate side than at the surface. For the sake of simplification, a -B-AB-type coating will subsequently be used to describe a -B(+A)-AB-type coating.
If the running direction of the strip is reversed, an -AB-B(+A)-type bilayer coating is then obtained, in which the AB sublayer is richer in element B on the substrate side than at the surface.
Referring to FIG. 1, if the B exit edge is placed at a distance hy from the path along which the strip runs (and not near it), upon exiting the deposition zone bxe2x80x2bxe2x80x3, mostly element A will be deposited and an -AB-A(+B) (or, more simply, -AB-A)-type bilayer coating will be obtained, in which the A(+B) layer will have a B content which will depend mainly on the flow rate of vapour from the source and on the residual pressure in the deposition chamber and in which the AB sublayer is richer in element A on the substrate side than at the surface.
If the running direction of the strip is reversed, a -B(+A)-AB-type bilayer coating is then obtained, in which the AB layer is richer in element B on the substrate side than at the surface.
It may therefore be seen that, in the running direction of the strip as indicated in FIGS. 1 and 2, such an arrangement of the edges of the useful emission window for elements A and B makes it possible to produce -B-AB-type and -AB-A-type bilayer coatings; by combining the two arrangements of A and B edges as illustrated in FIG. 3, -B-AB-A coatings are obtained in which the AB layer is always richer in element A on the substrate side than at the surface.
The direct effect of such an arrangement of the A and/or B edges, which consists in placing them at a distance hx and/or hy from the path along which the sheet runs, is to widen the two useful solid angles of emission of element A and of element B; increasing the distance hx from the A entry edge further increases the useful angle of emission of element B, the source of which is further away than that of element A, the source of which is closer, and therefore leads to the formation of a layer of element B beneath the layer of alloy AB; increasing the distance hy of the B exit edge further increases the useful angle of emission of element A, the source of which is further away than that of element B, the source of which is closer, and therefore results in the formation of a layer of element A on the layer of alloy AB; in general, increasing the distance of an edge further widens the useful angle of emission of an element whose source is further away and therefore results in the formation of a pure layer (or, in the majority of cases, weakly alloyed) of this element above or below the layer of alloy AB; the effect is all the more pronounced the greater the distance between the sources.
The object of the invention is to produce, apart from the coatings proposed in the document JP 06 212410 A, coatings of the A-AB-A type or the B-AB-B type, in which the AB layer is always richer in element A on the substrate side than at the surface if the strip runs from the A source towards the B source.
Of course, if the direction of the strip is reversed, it is possible to obtain the same types of coatings as before, but in this case the AB layer becomes richer in element B on the substrate side than at the surface.
For this purpose, the subject of the invention is a coating plant for obtaining a sheet, comprising, if A denotes chromium and if B denotes zinc:
an apparatus for running the sheet to be coated past a window for the evaporation or sublimation of element A and/or B,
a source for the evaporation or sublimation of element A and a source for the evaporation or sublimation of element B, these being placed successively in a direction parallel to that of the said running and in the direction of this running, so as to emit through the same window at a useful emission angle limited by the said window, the sheet to be coated running from an entry edge to an exit edge of the said window, characterized in that it comprises means for reducing the angle of emission from the source of element B below the limit represented by the exit edge, the said means for reducing the angle of emission from the source being mounted so as to move translationally perpendicular to the running of the sheet, it being possible for their position to vary between the source of element A and the source of element B so as to obtain either the -A-AB-A-type or the -B-AB-B-type coating.
The invention may also have the following features:
the plant also includes means for reducing the angle of emission from the source of element A below the limit represented by the exit edge, it being possible for the position of the said means to vary between the source of element A and the source of element B so as to obtain either the -A-AB-A-type or the -B-AB-B-type coating;
the said means for reducing the angle of emission are formed by at least one screen placed between the source of element A and the source of element B, it being possible for the position of the said means to vary between the source of element A and the source of element B so as to obtain either the -A-AB-A-type or the -B-AB-B-type coating.
In the particular case of coatings of Znxe2x80x94Cr alloy, reducing the angle of emission from the source of element Cr below the limit represented by the exit edge, a surface layer even richer in zinc is obtained, this being favourable to phosphatizability.
On the other hand, by reducing the angle of emission of the source of element Zn below the limit represented by the exit edge, a surface layer even richer in chromium is obtained, thereby favouring the direct adhesion of an organic coating.
According to a preferred embodiment of the invention, the said means for reducing the angle of emission are formed by a vertical movable screen placed between the source of element A and the source of element B.
Advantageously, it is possible, during the operation of the said process, for the positions of the vertical movable screen and of the entry and exit edges to be adjusted simultaneously. By doing this, the same thickness for the two sublayers is maintained even if the line speed, the level of the charges A and B in the crucibles of the sources or the evaporation rates are modified.
FIG. 4 illustrates in a non-limiting manner such a plant, which will be described in more detail later in the particular case of a coating comprising a main layer of zinc-chromium alloy.
The plant may obviously be used to coat continuous strips of sheet.
By virtue of this arrangement, it is possible to produce coatings comprising a layer of AB alloy richer in element A on the substrate side,
of the -A-AB type, by reducing the angle of emission from the B source below the limit represented by the A entry edge,
of the -AB-B type, by reducing the angle of emission from the A source below the limit represented by the B exit edge,
of the -A-AB-B type, by combining the above two means.
If a vertical movable screen is used, these various alternative embodiments may be obtained by varying the position of this screen between the source of element A and the source of element B or by using two screens.
Combining with the means described above and illustrated in FIGS. 1 to 3, it is therefore possible to produce multilayer coatings which are not achievable using the vacuum deposition apparatuses and processes described in the prior art, of the -A-AB-A type or of the -B-AB-B type, in which the layer of alloy AB is richer in element A on the substrate side.
The sheet according to the invention will now be described in more detail.
It is known that chromium crystallizes in an a body-centred cubic system, the cell parameter a0 of which is approximately 0.2884 nm (source: W. B. Pearson, Handbook of Lattice Spacings and Structure of Metals and Alloys, Pergammon Press); it is also known that a zinc-chromium alloy rich in chromium crystallizes in the same body-centred cubic system, with a larger cell parameter, the value a0xe2x80x2 of which is generally between 0.296 and 0.301 nm; the minimum amount of chromium needed to obtain a zinc-chromium alloy having this body-centred cubic structure depends on the conditions under which the alloy was prepared: referring to FIG. 5, which shows the diagram established by the Applicant from the amalgamation of tests for the identification and thermostability of the phases of Znxe2x80x94Cr alloys as a function of the chromium content (%) obtained by the PVD (Physical Vapour Deposition) process, it may be seen that the minimum weight content of chromium in order to obtain this xcex93 structure thus varied from approximately 20 to 34%, depending on these preparation conditions.
Because of the body-centred cubic structure of the subjacent layer of zinc-chromium alloy rich in chromium according to the invention, it has proved to be possible to obtain a partial epitaxial junction between this alloy and steel.
The expression xe2x80x9cpartial epitaxial junctionxe2x80x9d is understood to mean that, at the interface between the substrate and the subjacent layer of this alloy in the body-centred cubic form, there is continuity between some of the ferrite crystals of the steel at this interface and some of the body-centred cubic crystals of the Znxe2x80x94Cr alloy at this same interface.
Thus, according to the invention, by sandwiching a subjacent layer of Znxe2x80x94Cr alloy of body-centred cubic structure between a steel substrate and a main layer of Znxe2x80x94Cr alloy much leaner in chromium, which does not crystallize in the xcex93 body-centred cubic form and is intended to provide effective protection of the steel against corrosion, the adhesion of this main protective layer to the substrate is very greatly improved as long as a partial epitaxial junction between the steel and this interlayer is established.
One way of checking that the junction between the steel and this interlayer has a partial epitaxial nature consists in producing an electron diffraction pattern from thin foils obtained by cutting in the plane of the interlayer, near the interface with the steel.
FIG. 6 illustrates the pattern obtained when the junction is epitaxial in nature: it may be seen that the diffraction rings form interrupted lines and are not complete, indicating that the orientation of the body-centred cubic grains of the interlayer in the cutting plane is not random and that there is a partial epitaxial relationship with the grains located below the cutting plane, that is to say with the steel grains; in addition, the angle between the position of maximum intensity of the ring which corresponds to the {011} crystallographic planes of the xcex93-ZnCr phase and that of the {200} planes is 90xc2x0, thereby indicating a growth texture of the [011] type.
FIGS. 7A and 7B show, respectively:
the [011] (crystallographic notation) direction zone for a crystal lattice of the body-centred cubic type, of the type of that of the Znxe2x80x94Cr alloy rich in chromium of the interlayer according to the invention;
the preferred [011] orientation for the growth of the grains of this interlayer, during its deposition on the steel substrate.
It has been found that the subjacent layer according to the invention could significantly improve the adhesion of the coating as long as its thickness exceeds 0.01 xcexcm; very good results have been obtained with a thickness of approximately 0.02 xcexcm; since the xcex93 phase is brittle in nature, it is essential to prevent this layer from being too thick, in order to avoid problems when forming the coated sheet; thus, preferably the thickness of this layer is less than 1 xcexcm.
In particular, the invention is used to improve the adhesion of graded Znxe2x80x94Cr layers, the chromium content of which is greater in the depth than at the surface, such as the layers described in the abovementioned document JP 03-191053.
A variant of the invention consists in subjecting the sheet obtained to a heat treatment so as to transform, at least partly, the xcex4 phase of the main layer of the coating into the xcex6 phase, as described for example in document JP 08-013192 which indicates that such a treatment makes it possible to improve the resistance of the sheet to red rust.
In order to prepare a coated steel sheet according to the invention, it is preferred to use a vacuum deposition process since this type of process allows the proportions of chromium and zinc, which vary through the thickness of the coating: higher chromium content near the steel in order to form the adhesion interlayer according to the invention, lower chromium content near the surface, to be more easily controlled.
Another variant of the invention for transforming, at least partly, the xcex4 phase of the main layer of the coating into the xcex6 phase when the deposition is carried out under vacuum consists in carrying out the deposition on a substrate heated to a temperature above 170xc2x0 C.; by observation in section using a scanning electron microscope, it is found that the microstructure of the main layer of the coating is modified:
at 150xc2x0 C., a morphology similar to that obtained without preheating is observed: the main layer exhibits a columnar morphology, the crystals of which are oriented perpendicularly to the surface of the substrate or to the plane of the layer, and extend over the entire thickness of this layer (thickness about 5 xcexcm); the grain boundaries therefore pass right through the layer;
at 200xc2x0 C., the main layer exhibits a xe2x80x9cduplexxe2x80x9d morphology: from the surface as far as about mid-thickness of the layer, the morphology remains columnar whereas, further into the depth, the morphology is virtually equiaxed; X-ray diffraction analysis shows that the Znxe2x80x94Cr alloy deposited is partially transformed to the xcex6 phase. Under the following X-ray diffraction analysis conditions: X-ray tube, cobalt anticathode (30 kVxe2x88x9230 mA with Ka=0.179026 nm), rear monochromator and scintillation detector, grazing-incidence device (angle of incidence a=3xc2x0), the height of the (131) line of the xcex6-ZnCr phase is equal to or slightly less than the height of the (0002) line of the xcex4-ZnCr phase;
at 240xc2x0 C., the layer obtained is entirely transformed (equiaxed morphology) but has a low density because of the partial re-evaporation of the zinc under the vacuum deposition conditions.
Thus, according to this variant, which is aimed at obtaining a coating whose main layer is at least partially transformed to the xcex6 phase, the sheet to be coated is preferably heated between 170xc2x0 C. and 230xc2x0 C.; by heating the substrate in the approximately 170 to 200xc2x0 C. range, coatings are obtained whose main layer is two-phased, for which the height of the (131) line of the xcex6-ZnCr phase is between approximately 10 and 100% of the height of the (0002) line of the xcex4-ZnCr phase.
Without subsequent heat treatment, it is thus possible to produce a coating whose main layer has a xcex6 predominant phase.
By carrying out a suitable heat treatment after the deposition, it is possible, where appropriate, to completely transform the structure of the main layer into the xcex6 phase of equiaxed morphology having, over the entire thickness of this layer, grains of very small size, for example a size of between 10 and 200 nm.
One advantage of this variant of the invention is that a subsequent treatment of the type carried out for baking a coat of paint suffices to complete the transformation of a sufficient amount of the xcex4 phase into the xcex6 phase in order to obtain the best corrosion resistance; it is therefore no longer necessary to carry out a specific subsequent heat treatment in order to improve the corrosion resistance.
Since the xcex6 phase is predominant in the coating, an appreciable increase in corrosion protection provided by this coating is observed compared with the coating of the same thickness and the same composition but whose predominant phase is the xcex4 phase; this improvement stems, at least in part, from the microcrystalline structure obtained by heating the substrate during and/or after coating; indeed, the fact that grain boundaries no longer pass directly right through the layer, as in the case of the columnar morphology, could explain the appreciable improvement in the corrosion resistance.
The subjacent adhesion layer according to the invention is particularly advantageous because, even in the case of deposition on a heated substrate and/or in the case of subsequent heat treatments, the xcex93 phase of this subjacent layer is very temperature stable and continues to provide the specific effect of the invention of improving the adhesion of the coating, even after the xcex4 phase has been transformed into the xcex6 phase.